Why Do Mockingbirds Accept Invaders' Eggs?

http://youtu.be/qvx5JD3VyxI In the form of brood parasites, the bird world has enough irresponsible moms to start a reality TV show: cowbirds, for instance, lay their eggs in other species' nests, stab most of the hosts' eggs to death, and then leave their offspring to be raised by the host parents. The standing explanation for this involves most host birds being not that sharp on the uptake (watch a tiny warbler fussing over a cuckoo chick ten times its size (above) and you'd think that too). But maybe, a new study suggests, it's sometimes to the host's benefit to let imposter eggs stay in their nests. The researchers chose mockingbirds as their hosts and cowbirds as their parasites, because mockingbirds usually fight like crazy to keep cowbirds of their nests but get strangely quiescent once the invaders have laid their eggs, a behavior that piqued the researchers' interest. Once all the birds in the sample population had laid, the researchers went around adding and removing eggs from nests to see whether having a certain number of cowbird eggs affected mockingbird survival. They found that mockingbird eggs that shared their digs with cowbird eggs and suffered repeated cowbird invasions were more likely to survive, apparently because when each cowbird arrived, it would stab a certain proportion of the eggs in the nest regardless of whether they were host eggs or the eggs of the previous cowbird. Letting the parasite's eggs stay, then, means that more of the host eggs avoid getting stabbed. The researchers conclude that when there are a lot of cowbirds around and hence a high probability of multiple nest hijackings, it makes sense for mockingbird parents not to shove out the invaders' eggs. Nice. And with all this dubious parenting and wanton violence, it's straight from an episode of Teen Mom meets Cops, no?